The stock exhaust is already tuned for low noise and reduced emissions. The back pressure helps generate strong low end to midrange torque.
An aftermarket exhaust makes the gas flow straighter and faster by reducing the number of internal noise dampening baffles, straightening and smoothing exhaust pipes, and leaving out the catalytic converter. High rpm flow improves. Without further tuning to the fuel delivery, the lack of back pressure may reduce low rpm torque!
Guile, what do you want to achieve through 'tuning' the stock exhaust? How do you fast and how hard do you ride your Xciting? Where do you expect improvement?